Electricity is a staple of life. Nearly sixty percent (60%) of the United States' energy supply comes from oil and coal, both of which have finite availability, are harmful to the environment (e.g., global warming), and experience wide price fluctuations. Nuclear energy, which accounts for roughly 9% of the U.S. energy supply, has been viewed with scrutiny since the Three Mile Island meltdown in 1979 and the Chemobyl disaster in 1986, and safety concerns have again been raised in view of the 2011 nuclear reactor meltdowns in Japan.
There is an ever-growing need for energy, and renewable energy sources are increasingly being considered as a possible solution. In a May 2011 report, the Intergovernmental Panel on Climate Change set a global goal pursuant to which seventy-seven percent (77%) of the world's energy will be supplied from renewable sources by 2050. At the time of the report, only approximately thirteen percent (13%) of existing global energy sources used for generating electricity were renewable. In order to dramatically increase the world's use of renewable sources of electricity, multiple renewable energy sources will need to be tapped. Wind and solar energy are the most prominent renewable sources currently being used and developed, but neither will be sufficient to meet the seventy-seven percent (77%) goal on their own or in combination with the other by 2050. The use of wind and solar sources also faces significant potential limitations. Harvesting wind and solar energy to meet electricity consumption demands presents multiple practical challenges, including that each is weather and location dependent, requires relatively expensive and bulky equipment, and often must be located remotely from population centers where the electricity is most needed.
Another potentially valuable renewable energy source for generating electricity is wastewater. Wastewater includes the everyday liquid waste discharged by residential commercial and industrial buildings, including homes and apartment buildings, industrial plans, and agricultural facilities during normal, everyday use. Household wastewater is produced by showers, dish washers, toilets, laundry machines, and other sources. Currently, wastewater is a significantly underutilized potential source of electricity and conventional wastewater treatment methods consume more energy than they produce.
Microbial fuel cells (MFCs) have been considered for electricity generation using wastewater sources. Typically, microbial fuel cells convert chemical energy into electrical energy through a catalytic reaction, in which microorganisms such as bacteria are used as a catalyst in an anode compartment of the fuel cell. Typically, the microorganisms catabolize organic matter (or “biomass”) contained in the wastewater under anaerobic conditions to produce carbon dioxide, protons, and electrons. For example, MFCs generate electricity by using an electrode as the electron receptor in the metabolism of a group of bacteria called exoelectrogens. These bacteria consume dissolved organic matter such as glucose:C6H12O6+6H2O→6CO2+24H++24e−  (1)
The electrons are then used to reduce Nicotinamide Adenine Dinucleotides (NAD) to NADH, the principal energy-transfer molecule in the cell:e−+H++NAD−→NADH  (2)
The resultant NADH molecules in turn transfer the electrons to an electrical circuit, providing current for use in the circuit. More specifically, the electrons absorbed by an anode in this manner are conducted through the circuit and are transferred to a cathode compartment of the fuel cell containing a cathode connected to the electrical circuit. At the cathode electrons provide the energy needed to drive the reaction between protons and oxygen to form water:4e−+4H++O2→2H2O  (3)
The protons at the cathode are supplied by the initial oxidation of the dissolved organic matter and migrate to the cathode via one or more processes. Often, the proton transport is achieved via diffusion of the protons through a cation exchange membrane separating the anode and cathode compartments of the microbial fuel cell. The potential across the circuit is driven by the reaction between the protons and oxygen, usually under aerobic conditions, at the cathode to form water. This reaction optionally may be catalyzed via microbial activity or precious metals at the cathode.
The potential chemical energy or cell voltage available from the reactions inside a microbial fuel cell is thermodynamically fixed based on the concentration of reactants and the temperature of the medium, according to the Nernst equation. The total energy available from the fuel cell is the difference between the cathode and anode voltages:Eemf=Ecathode+Eanode  (4)
This is essentially the potential for the oxidation of the organic matter, e.g., glucose:EGluc=E0−RT/24F·ln(([CO2]2[H+]24)/([H2O]6[C6H12O6]))  (5)                wherein R is the ideal gas constant, T is the temperature of reaction, F is Faraday's constant [C/mole-e−], and E0 is standard potential at pH 1 and 25° C.        
The energy from glucose oxidation is used for the reduction of NAD:ENAD=E0−RT/2F·ln([NADH]/([H+][NAD−]))  (6)
NAD oxidation then releases electrons to the circuit, which drive the reaction at the cathodeEO2=E0−RT/4F·ln([H2O]2/([H+]4[O2]))  (7)
The consumption rate of BOD in the MFC is a function of microbial kinetics, substrate concentration, and reactor architecture. For a plug-flow reactor typically used in wastewater treatment, the residence time of water in the reactor, given a required BOD reduction is a function of all these parameters:
                    θ        =                              1            q                    ⁢                      {                                                            (                                                            K                                              X                        +                                                  YS                                                                                                                                            ⁢                            i                                                                                                                +                                          1                      Y                                                        )                                ⁢                                  ln                  ⁡                                      (                                          X                      +                                              YS                                                                                                                                  ⁢                          i                                                                    -                                              YS                                                                                                                                  ⁢                          e                                                                                      )                                                              -                                                (                                      K                                          X                      +                                              YS                                                                                                                                  ⁢                          i                                                                                                      )                                ⁢                                  ln                  ⁡                                      [                                                                                            S                                                                                                                                            ⁢                            e                                                                          ⁢                        X                                                                    S                                                                                                                                  ⁢                          i                                                                                      ]                                                              -                                                1                  Y                                ⁢                                  ln                  ⁡                                      (                    X                    )                                                                        }                                              (        8        )                            wherein Θ is hydraulic residence time [d], K is half saturation coefficient [mg BOD/L], X is mixed liquor suspended solids [mg/L], Si is influent BOD [mg BOD/L], Se is effluent BOD [mg BOD/L], Y is max yield [g VSS/g BOD], and q is max substrate utilization [mg/mg VSS−d].        
The current generated by the biofilm on the anode is a function of the microbial kinetics, the concentration of bacteria and the energy content of the substrateI=(μmaxX*be/Y)·(FCE)  (9)                wherein I is current, μmax is maximum specific growth rate of bacteria, be is electrons per mole of substrate, X is active bacteria [mg/L], Y is max yield [g VSS/g BOD], F is Faraday's constant [C/mol-e−], and CE is columbic efficiency, ratio of electrons transferred to anode relative to total liberated.        
The impact of temperature on the kinetics varies according to the bacterial culture. Correlations for scaling microbial activity according to temperature may be used with empirical data:k2=k1eφ(T2−T1)  (10)                wherein k is kinetics parameter T and K (defined above), and φ is an adjustment factor.        
Lab-scale microbial fuel cells operating at optimum conditions have been found to create power densities ranging from 5 to 1,000 Watts per cubic meter (W/m3) of reactor volume. Such power densities, particularly at the lower region of the range, have limited commercial uses.